Thermosiphon with flexible boiler plate

ABSTRACT

A thermosiphon cooling assembly includes a housing having a lower portion and an upper portion. A refrigerant is disposed in the lower portion for liquid-to-vapor transformation. A boiler plate defines an outer bottom wall of the housing for transferring heat from the electronic device to the refrigerant. This transfer of heat to the refrigerant leads to a liquid-to-vapor transformation and a pressure build up in the housing. The boiler plate is flexible for reacting with the electronic device in response to the pressure build up in the housing. The boiler plate includes an inflexible first area for overlying the electronic device and a flexible second area which surrounds the first area of the boiler plate. The second area has a thickness t 2  less than the thickness t, of the first area thus allowing the boiler plate to flex in the second area.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The subject invention relates to a thermosiphon cooling assembly for cooling a flat electronic device disposed on a support.

2. Description of the Prior Art

The operating speed of computers is constantly being improved to create faster computers. With this, comes increased heat generation and a need to effectively dissipate that heat.

Heat exchangers and heat sink assemblies have been used that apply natural or forced convection cooling methods to dissipate heat from electronic devices that are highly concentrated heat sources such as microprocessors and computer chips. These heat exchangers typically use air to directly remove heat from the electronic devices; however air has a relatively low heat capacity. Thus, liquid-cooled units called LCUs employing a cold plate in conjunction with high heat capacity fluids have been used to remove heat from these types of heat sources. Although LCUs are satisfactory for moderate heat flux, increasing computing speeds have required more effective heat sink assemblies.

Accordingly, thermosiphon cooling units (TCUs) have been used for cooling electronic devices having a high heat flux. A typical TCU absorbs heat generated by the electronic device by vaporizing the working fluid housed on the boiler plate of the unit. The boiling of the working fluid constitutes a phase change from liquid-to-vapor state and as such the working fluid of the TCU is considered to be a two-phase fluid. The vapor generated during boiling of the working fluid is then transferred to a condenser, where it is liquefied by the process of film condensation over the condensing surface of the TCU. The heat is rejected into a stream of air flowing through a tube running through the condenser or flowing over fins extending from the condenser. Alternatively, a second refrigerant can flow through the tube increasing the cooling efficiency. The condensed liquid is returned back to the boiler plate by gravity to continue the boiling-condensing cycle.

An example of a cooling system for electronic devices is disclosed in U.S. Pat. No. 6,588,498 to Reyzin et al.

The Reyzin patent discloses an assembly for cooling an electronic device including a hermetically sealed housing having a lower portion and an upper portion. A refrigerant is disposed in the lower portion. Heat generated by the electronic device dissipates into the housing through a boiler plate causing the refrigerant to boil. The vapor boiled off the refrigerant then rises upwardly into the upper portion of the housing. The vapor in the upper portion of the housing is condensed by the movement of air through a plurality of radiation chambers and over a plurality of fins extending between the radiation chambers and along the side walls of the housing.

Although the prior art dissipates heat from electronic devices, as computing speeds increase, there is a continuing need for cooling devices having more efficient or alternative heat transfer capabilities as compared to the conventional electronic cooling assemblies.

SUMMARY OF THE INVENTION AND ADVANTAGES

The invention provides a thermosiphon cooling assembly for cooling a flat electronic device disposed on a support. The assembly includes a hermetically sealed housing that defines a lower portion and an upper portion, with a refrigerant disposed in the lower portion of the housing for liquid-to-vapor transformation. A boiler plate defines an outer wall of the housing and transfers heat from the electronic device to the refrigerant for liquid-to-vapor transformation and a pressure build up in the housing. The assembly is distinguished by the boiler plate being flexible for reacting with the electronic device in response to the pressure build up in the housing.

The subject invention also provides a method of cooling a flat electronic device disposed on a support wherein heat is generated from the electronic device and is transferred from the electronic device to the boiler plate. The method includes transforming a refrigerant disposed in the lower portion of the housing from liquid-to-vapor and increasing a pressure in the housing as heat is transferred from the electronic device. The method is distinguished by flexing the boiler plate to react with the electronic device in response to increasing the pressure in the housing.

Accordingly, the invention is highly responsive to the thermal load imposed on the boiler plate by the electronic device. This thermal response is then utilized to alter the thermal resistance of an intervening grease layer that bonds the electronic device to the boiler plate. The boiler plate flexes as the refrigerant vapor pressure within the housing changes depending on the thermal load of the electronic device. The flexing of the boiler plate is further facilitated by a biasing mechanism supporting the boiler plate for movement relative to the electronic device.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is a perspective view of a thermosiphon and an air moving device embodying the present invention; and

FIG. 2 is an elevational cross-sectional view taken along line 2-2 of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the Figures, wherein like numerals indicate corresponding parts throughout the several views, a thermosiphon cooling assembly 20 is generally shown for cooling a flat electronic device 22 disposed on a support 24 in FIGS. 1 and 2.

The assembly 20 comprises a housing 26 generally indicated which is generally cubical and hermetically sealed. The housing 26 defines a generally rectangular lower portion 28 and an upper portion 30. A refrigerant 32 is disposed in the lower portion 28 for liquid-to-vapor transformation.

The upper portion 30 includes a plurality of spaced radiation chambers 34 extending through the housing 26 and a plurality of cooling fins 36 disposed within the radiation chambers 34. The radiation chambers 34 are separated in the upper portion 30 of the housing 26 by condenser fingers 38 that extend upwardly in the upper portion 30 from an open space above the liquid refrigerant 32 for receiving the vapor from the refrigerant 32. The condenser fingers 38 are also generally rectangular.

A boiler plate 40 generally indicated defines an outer or bottom wall of the housing 26 for transferring heat from the electronic device 22 to the refrigerant 32. This transfer of heat to the refrigerant 32 leads to a liquid-to-vapor transformation and a pressure build up in the housing 26. The boiler plate 40 includes a first area 42 for overlying the electronic device 22 and a second area 44 which surrounds the first area 42 of the boiler plate 40. The second area 44 has a thickness t₂ less than the thickness t, of the first area 42. A plurality of heat transfer fins 46 are disposed on the first area 42 in the lower portion 28 of the housing 26 for enhancing heat transfer to the refrigerant 32.

A grease layer 48 is disposed between the first area 42 and the electronic device 22 for enhancing the thermal interface between the boiler plate 40 and the electronic device 22. The grease layer 48 fills the microscopic spaces that are created when the first area 42 of the boiler plate 40 is placed into contact with electronic device 22. No two surfaces are perfectly flat and these surfaces contain many microscopic spaces and the grease layer 48 is utilized to fill these spaces.

The assembly 20 is distinguished by the boiler plate 40 being flexible for reacting with the electronic device 22 in response to the pressure build up in the housing 26. This is accomplished by the first area 42 of the boiler plate 40 being inflexible and the second area 44 of the boiler plate 40 being flexible. The thickness t₂ of the second area 44, being less than the thickness t, of the first area 42 of the boiler plate 40 and of a thin material, allows the boiler plate 40 to flex in the second area 44. The second area 44 may comprise a thin gage metal, and/or various materials that are inert to the refrigerant 32. The first area 42 along with rest of the housing 26 may comprise a thicker metal that will not flex, e.g., rigid, in response to the pressure build up in the housing 26.

The assembly 20 is highly responsive to the instantaneous thermal load imposed on the boiler plate 40 by the electronic device 22. As the refrigerant 32 is heated by the electronic device 22 through the boiler plate 40 and the heat transfer fins 46 the thermal load imposed on the assembly 20 increases. Also as the refrigerant 32 is heated, the liquid refrigerant 32 changes to a pressurized vapor. This pressurized vapor moves upwardly into upper portion 30. The increased pressure in the housing 26 causes the housing 26 to want to expand outwardly. The thin walls of the second area 44 allow the housing 26 to expand in the second area 44 in response to the pressure in the housing 26. As the second area 44 expands outwardly from the housing 26, the inflexible and flat first area 42 is pushed toward the electronic device 22. As the pressurized vapor in the housing 26 increases due to the higher thermal load imposed by the electronic device 22 the pressure on the grease layer 48 increases forcing some grease out. This lowers or reduces the grease layer 48 thickness and consequently its thermal resistance is lowered making the assembly 20 more efficient.

An air moving device 50, more specifically a cooling fan, moves air through the radiation chambers 34 and over the cooling fins 36. The air flow dissipates the heat generated by the electronic device 22 and condenses the pressurized vapor in the upper portion 30. As the pressurized vapor is condensed the cooled liquid refrigerant 32 returns to the lower portion 28 of the housing 26.

A grease reservoir 52 is disposed on the first area 42 of the boiler plate 40 and surrounds the electronic device 22 for receiving grease from the grease layer 48 as the boiler plate 40 reacts with the electronic device 22. As the pressurized vapor in the housing 26 increases due to the higher thermal load imposed by the electronic device 22 grease from the grease layer 48 will be pushed out from in between the electronic device 22 and the boiler plate 40 and into the grease reservoir 52. As the pressurized vapor in the housing 26 decreases due to a lower thermal load imposed by the electronic device 22, the pressure on the grease layer 48 decreases and allows for grease from the grease reservoir 52 to be sucked and moved back into the intervening space between the boiler plate 40 and the electronic device 22.

The assembly 20 further includes a biasing mechanism 54 generally indicated for connection to the support 24 to facilitate movement of the housing 26 as the boiler plate 40 reacts with the electronic device 22. The housing 26 includes flanges 56 that are disposed on opposite sides of housing 26. The biasing mechanism 54 includes a coil spring 58 that reacts with each of the flanges 56. A connector 60, illustrated as a bolt, interconnects each of the coil springs 58 and the support 24 for compressing each of the coil springs 58 in response to movement of the housing 26 away from the electronic device 22. The assembly 20 may include a plurality of biasing mechanisms 54 as shown in FIG. 1. In addition, the housing 26 may include a plurality of biasing mechanisms 54 on each side of the housing 26. The biasing mechanisms 54 may be utilized to facilitate the movement of the air moving device 50 with the movement of the housing 26 as shown in FIG. 1. The air moving device 50 may be attached to the housing 26 as shown in FIG. 1, or the air moving may be a separate unit.

The subject invention also provides for a method of cooling an electronic device 22 by disposing a refrigerant 32 in the lower portion 28 of the housing 26 for liquid-to-vapor transformation and a pressure build up in the housing 26. The method comprises the steps of generating heat by the electronic device 22, and transferring heat from the electronic device 22 through the boiler plate 40 to the refrigerant 32. The method proceeds with the steps of transforming the refrigerant 32 disposed in the lower portion 28 of the housing 26 from liquid-to-vapor and increasing the pressure in the housing 26 as heat is transferred from the electronic device 22. The method is distinguished by the boiler plate 40 being flexible to react with the electronic device 22 in response to increasing the pressure in the housing 26. The method is more specific by supporting the housing 26 for movement relative to the electronic device 22 and biasing against such movement of the housing 26 away from the electronic device 22. This biasing may be accomplished through the use of the coil spring 58.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. The invention may be practiced otherwise than as specifically described within the scope of the appended claims. 

1. A thermosiphon cooling assembly for cooling a flat electronic device disposed on a support and comprising; a housing being hermetically sealed and defining a lower portion and an upper portion, a refrigerant disposed in said lower portion of said housing for liquid-to-vapor transformation, and a boiler plate defining an outer wall of said housing for transferring heat from the electronic device to said refrigerant for liquid-to-vapor transformation and a pressure build up in said housing, said boiler plate being flexible for reacting with the electronic device in response to said pressure build up in said housing.
 2. An assembly as set forth in claim 1 including a biasing mechanism for connection to the support to facilitate movement of said housing as said boiler plate reacts with the electronic device.
 3. An assembly as set forth in claim 1 wherein said boiler plate includes a first area being inflexible for overlying the electronic device and a second area being flexible in a thickness less than the thickness of said first area of said boiler plate for allowing said boiler plate to flex in said second area.
 4. An assembly as set forth in claim 3 including a grease reservoir disposed on said boiler plate to surround the electronic device for receiving grease from a grease layer disposed between said first area and the electronic device as said boiler plate reacts with the electronic device.
 5. An assembly as set forth in claim 3 wherein said second area surrounds said first area.
 6. An assembly as set forth in claim 5 including a plurality of heat transfer fins disposed on said first area in said lower portion of said housing.
 7. An assembly as set forth in claim 3 including a biasing mechanism for connection to the support to facilitate movement of said housing as said boiler plate reacts with the electronic device.
 8. An assembly as set forth in claim 7 wherein said biasing mechanism includes at least one coil spring for compressing in response to movement of said housing away from the electronic device.
 9. An assembly as set forth in claim 8 wherein said housing includes flanges disposed on opposite sides of said housing, one of said coil springs reacting with each of said flanges, and a connector for interconnecting each of said coil springs and the support for compressing each of said coil springs in response to movement of said housing away from the electronic device.
 10. An assembly as set forth in claim 7 including a grease reservoir disposed on said first area to surround the electronic device for receiving grease from a grease layer disposed between said first area and the electronic device as said boiler plate reacts with the electronic device.
 11. An assembly as set forth in claim 10 including the grease layer disposed on said first area of said boiler plate and in said reservoir for enhancing the thermal interface between said boiler plate and the electronic device.
 12. An assembly as set forth in claim 10 wherein said upper portion includes a plurality of spaced radiation chambers extending through said housing and a plurality of cooling fins disposed within said radiation chambers.
 13. An assembly as set forth in claim 12 wherein said housing is generally cubical.
 14. An assembly as set forth in claim 13 including an air moving device for moving air through said radiation chambers and over said cooling fins.
 15. A thermosiphon cooling assembly for cooling a flat electronic device disposed on a support and comprising; a housing being generally cubical and hermetically sealed and defining a lower portion and an upper portion with said upper portion having a plurality of spaced radiation chambers extending through said housing, a plurality of cooling fins disposed within said radiation chambers, an air moving device for moving air through said radiation chambers and over said cooling fins, a refrigerant disposed in said lower portion of said housing for liquid-to-vapor transformation, a boiler plate defining a bottom wall of said housing for transferring heat from the electronic device to said refrigerant for liquid-to-vapor transformation and a pressure build up in said housing, said boiler plate having a first area for overlying the electronic device and a second area surrounding said first area of said boiler plate, a grease layer disposed between said first area and the electronic device for enhancing the thermal interface between said boiler plate and the electronic device, a plurality of heat transfer fins disposed on said first area in said lower portion of said housing for enhancing heat transfer to said refrigerant, said boiler plate being flexible for reacting with the electronic device in response to said pressure build up in said housing by said first area of said boiler plate being inflexible and said second area of said boiler plate being flexible in a thickness less than the thickness of said first area of said boiler plate for allowing said boiler plate to flex in said second area, a grease reservoir disposed on said first area of said boiler plate to surround the electronic device for receiving grease from said grease layer as said boiler plate reacts with the electronic device, a biasing mechanism for connection to the support to facilitate movement of said housing as said boiler plate reacts with the electronic device, said housing including flanges disposed on opposite sides of said housing, said biasing mechanism including a coil spring reacting with each of said flanges, and a connector for interconnecting each of said coil springs and the support for compressing each of said coil springs in response to movement of said housing away from the electronic device.
 16. A method of cooling a flat electronic device disposed on a support comprising the steps of; generating heat by the electronic device, transferring heat from the electronic device to a boiler plate defining an outer wall of a housing being hermetically sealed with the housing defining a lower portion and an upper portion, transforming a refrigerant disposed in the lower portion of the housing from liquid-to-vapor and increasing a pressure in the housing in response to heat being transferred from the electronic device, and flexing the boiler plate to react with the electronic device in response to increasing the pressure in the housing.
 17. A method as set forth in claim 16 further including supporting the housing for movement relative to the electronic device and biasing against such movement of the housing away from the electronic device. 